The present invention relates to power amplifiers. In particular, the present invention relates to efficient amplification of signals at a non-peaked average power level whose envelope is amplitude modulated, and specifically the application of linear amplification at RF frequencies of such signals up to high power levels.
The requirements for higher data rates and bandwidth efficiency in wireless communication standards has caused designers to implement varying envelope modulation formats. The varying AM content of these formats allows for additional information to be transmitted in a given bandwidth. This varying envelope significantly constrains the ability of the power amplifier in a wireless device to linearly transmit the waveform. In addition, the varying envelope exhibits a xe2x80x9cpeak-to-averagexe2x80x9d ratio, where the average envelope power may be significantly below the peak envelope power. Ideally, the power amplifier would be capable of delivering peak power while operating at a much lower average power. Attempts at this, however, typically result in the application xe2x80x9cbacking-offxe2x80x9d the power amplifier from peak in order to avoid clipping and distortion of the output waveform. This also results in an associated large decrease in efficiency from the peak. Moreover, the linearity of the power amplifier often requires that its output power be backed off even further than PSAT ((dBm)-PeakToAvgRatio(dB)) in order to satisfy further constraints in adjacent channel power rejection (ACPR) and error vector magnitude (EVM). At a minimum, the power amplifier must provide xe2x80x9cheadroomxe2x80x9d for the output power to actually reach the peak output power and not saturate prematurely. It thus becomes critical for efficiency that the power amplifier topology be able to maintain efficiency at higher power levels.
One established technique for extending peak efficiency under xe2x80x9cbacked-offxe2x80x9d power conditions is the xe2x80x9cDohertyxe2x80x9d amplifier, an example of which is shown in the schematic diagram of FIG. 1. The Doherty arrangement 2 utilizes two power amplifiers 4, 6 that saturate at different input power levels. This allows one amplifier to reach saturation and peak efficiency before the second. Because power amplifiers with higher output load impedances saturate at lower output power levels, the dynamic loading of the first power amplifier by a second amplifier can change the output power level at which it saturates and achieves peak efficiency. This characteristic allows it to behave in a saturated mode over a range of output power because its load impedance decreases along with increasing output power. This dynamic loading and the variation in output impedance over a range of output powers are achieved with a special power-combining load network 8.
In the typical exemplary implementation as shown, this output power combiner 8 utilizes a quarter-wave transformer 10 with characteristic impedance of 35.36 Ohms to transform from the load resistance of 50 to 25 at the output combiner node 8. A second quarter-wave transformer 12 with characteristic impedance of 50 Ohms transforms the output combiner node impedance to the impedance seen at the output of the carrier power amplifier 4. In order to correct for the phase difference between the paths of the two amplifiers 4, 6, a quarter-wave transmission line 14 is inserted at the input of the peaking amplifier 6, so that they sum coherently at their outputs. Initially, with the peaking amplifier 6 OFF, the load network 16 presents to the carrier power amplifier 4 in FIG. 1 a load impedance of 2*ROPT=100 Ohms, and the high impedance of the peaking power amplifier does not significantly load the output combiner node.
The higher impedance presented to the carrier power amplifier 4 forces it to saturate earlier to optimum load impedance. In the exemplary case, shown in FIG. 1, the 2*ROPT load forces it to saturate 3 dB below peak. Once the peaking amplifier 6 activates, its finite output impedance starts to decrease the load impedance of the carrier power amplifier 4, until the point when both the carrier and peaking amplifiers 4, 6 deliver equal power into their own respective local load impedances of ROPT=50 Ohms.
The effect of two power amplifiers delivering equally is that they provide 3 dB more output power than one alone, and given the 3 dB early saturation of the carrier amplifier 4, the pair 4, 6 provides a net 6 dB power range over which efficiency is maintained at nearly peak saturated efficiency. This power range is critical xe2x80x9cheadroomxe2x80x9d required for the envelope variation to be faithfully recreated at the power amplifier output, while still maintaing peak saturated efficiency.
Previous implementations of the Doherty configuration have utilized a class B amplifier as the carrier amplifier, and a class C amplifier as the peaking amplifier as shown in FIG. 1. The class C peaking amplifier is used because of its unique property of only turning xe2x80x9cONxe2x80x9d once a threshold input power is delivered to it. This characteristic makes it convenient to drive both power amplifiers and to use this threshold property to turn on the peaking amplifier at the point that the carrier amplifier saturates.